The role of Carbon Dioxide Removal (CDR) technologies (sometimes called Negative Emissions Technologies, or NETS) in achieving the Paris Agreement goal of reaching net zero carbon dioxide emissions by 2050 is increasingly recognised in scientific and public debate. For example, the IPCC writes ‘[t]he deployment of CDR to counterbalance hard-to-abate residual emissions is unavoidable if net zero carbon dioxide or greenhouse gas emissions are to be achieved’ (IPCC, 2022, 36). It seems that CDR technologies could be a key part of the effort to keep global temperature increases to a manageable level. Accelerated by the Paris Agreement’s affirmation of the 2°C temperature target and acknowledgement of an aspirational 1.5°C target, the possibility of removing carbon and thereby producing ‘negative emissions’ gained significant traction. There is increasing academic research into the technical feasibility of various CDR technologies, as well as some attempts to develop CDR techniques by private companies. Some activities for example, now-defunct US company Running Tide, the Canadian company Planetary Technologies, the Haida Gwaii ‘Salmon Restoration project’, and the LOHAFEX project (for discussion of the latter two, see respectively Buck, Reference Buck, Blackstock and Low2018; White, Reference White2018; Kintisch, Reference Kintisch2010). Accordingly, governance of CDR technologies is an increasing area of research and debate, and it is ethical questions that are at the heart of many governance issues (for discussion, see Honegger et al., Reference Honegger, Baatz, Eberenz, Holland-Cruz, Michaelowa, Pokorny, Poralla and Winkler2022).

Among the many ethical objections that can be levelled against CDRs, three concerns have become the most prominent in scientific discussions. First, CDR may create a ‘moral hazard’, undermining emissions reduction (Shue, Reference Shue2017, following Anderson & Peters, Reference Anderson and Peters2016; for an in-depth review of research on CDR and moral hazard, see Carton et al., Reference Carton, Hougaard, Markusson and Lund2023). The introduction of CDR within economic models of climate mitigation obstructs near-term mitigation because CDR lowers the aggregate costs of mitigation over the century, while the availability of CDR in models may provide policymakers with an excuse for slower emissions cuts (Lenzi, Reference Lenzi2018, see also Hollnaicher, Reference Hollnaicher2022).

The second concern is that planning to rely upon the ability to scale up CDR to the levels seen in many modelled mitigation scenarios risks a catastrophic policy failure to stabilise the average global temperature at 2°C or below (Fuss et al., Reference Fuss, Canadell, Peters, Tavoni, Andrew, Ciais, Jackson, Jones, Kraxner, Nakicenovic, Le Quéré, Raupach, Sharifi, Smith and Yamagata2014; Shue, Reference Shue2017). Some commentators therefore regard ‘betting on NETS’ (Fuss et al., Reference Fuss, Canadell, Peters, Tavoni, Andrew, Ciais, Jackson, Jones, Kraxner, Nakicenovic, Le Quéré, Raupach, Sharifi, Smith and Yamagata2014) as an ‘unjust gamble’ with the welfare of future generations (Shue, Reference Shue2017). Others regard the development of CDRs as a prudent measure that could be a useful part of the fight against climate change, whilst acknowledging that emission reductions is the preferred strategy (Moellendorf, Reference Moellendorf2022).

One potential reason for any such policy failure is connected to the third set of ethical concerns. There are potential harmful side-effects of CDR, particularly in the case of very large-scale implementation. The main worry here is that such side-effects could be severely unjust (Shue, Reference Shue2017, Reference Shue2021; Lenzi, Reference Lenzi2018; Lenzi et al., Reference Lenzi, Schübel and Wallimann-Helmer2023; Minx et al., Reference Minx, Lamb, Callaghan, Fuss, Hilaire, Creutzig, Amann, Beringer, de Oliveira Garcia, Hartmann, Khanna, Lenzi, Luderer, Nemet, Rogelj, Smith, Vicente Vicente, Wilcox and del Mar Zamora Dominguez2018). Most attention has been paid to the biomass-based BECCS technique. Typical mitigation scenarios featuring BECCS have been alleged to require a land area the size of India, while earlier climate models predicted the expansion of biomass to occur predominantly in tropical regions (Anderson & Peters, Reference Anderson and Peters2016). Such possibilities for implementation could create serious threats of injustice related to local food security, water availability, and biodiversity (Shue, Reference Shue2017). Accordingly, ‘justice in siting’ (Rayner et al., Reference Rayner, Heyward, Kruger, Redgewell and Savulesu2013) and navigating trade-offs between space and resources (Low et al., Reference Low, Boettcher, Asayama, Baum, Borth, Brown, Clingerman, Dauvergne, De Pryck, Gupta, Honegger, Lenzi, Reitsma, Schenuit, Scott-Buechler and Valenzuela2024) are increasingly important issues.

This collection aims to contribute to the discussion about the permissible use and governance of CDR technologies by both analysing some of the ethical challenges of modelling CDR technologies and discussing the fairness of potential implementations in practice. All of them go beyond the overwhelming focus on ‘moral hazard’ which characterised the earliest interventions on the ethics of CDR. The first two articles both question the common idea that the development of CDRs is ‘necessary’ to meet the Paris Agreement goals, by challenging common assumptions made in economic modelling. The third and fourth articles each consider a problem that could arise if CDR technologies are used.

Simon Hollnaicher asks how CDRs should be modelled to serve as a good scientific basis for policy advice. He identifies a bias towards CDR in integrated assessment models (IAMs). He first distinguishes between ‘false positives’, where the potential of large-scale CDR is overestimated and ‘false negatives’ where it is underestimated. The consequences of overestimating the potential of CDR are far more serious, from an ethical perspective, than the consequences of underestimating it and so, he argues optimistic assumptions about the potential of large-scale CDR demand a greater burden of proof. IAMs, he argues, include a great deal of CDR and therefore contain a ‘normative bias’ in that certain results are systematically favoured over others, in this case, a bias in favour of market-based measures and technological measures such as CDRs. He identifies three elements in IAMs that play an especially influential role in creating this systemic bias: idealised implementation, perfect foresight, and high intertemporal discount rates. Hollnaicher concludes by supporting Carrier’s (Reference Carrier2022) two strategies for dealing with values in policy-relevant science. The first is transparency: of making value judgements explicit. Whilst there have been some high-profile cases which have highlighted this need (e.g. in discounting), Hollnaicher argues that there is much work to be done when it comes to IAMs. In particular, discount rates are rarely varied in IAMs and it can be difficult even to identify certain value judgements. The second strategy would be to embrace plurality concerning values, and to observe their implications for a wider range of technological, institutional and behavioural changes. Hollnaicher suggests that this approach goes beyond plugging in different value positions into existing modelling scenarios and instead involves much more dedicated interdisciplinary work in the development of alternative scenarios.

Given the modelling bias in favour of CDR technologies, it is even more important to consider alternative scenarios, and the values underlying the selection of these scenarios. Voget-Kleschin et al. (Reference Voget-Kleschin, Baatz, Heyward, Van Vuuren and Mengis2024) explore the commonly supposed choice between large-scale CDR and technologically intense mitigation pathways and argue that this obscures other viable alternatives. They argue that pathways involving lifestyle changes or population policies rarely feature in scientific conversations about climate change mitigation, as if there was a self-imposed ‘taboo’ against discussing such options. They undertake an ethical analysis of four alternative mitigation pathways proposed by Van Vuren et al. (Reference van Vuuren, Stehfest, Gernaat, van den Berg, Bijl, de Boer, Daioglou, Doelman, Edelenbosch, Harmsen, Hof and van Sluisveld2018), namely ‘life-style changes’ (i.e. reduced carbon dioxide consumption by consumers), ‘low population’, ‘low non-CO₂’, (replacing meat products with alternatives) and ‘agricultural intensification’ (increasing livestock yields in lower-income countries). Their overarching conclusion is that the moral permissibility of each pathway depends on the precise implementation of measures rather than the kind of measures highlighted. A corollary of this is that the scenarios that are commonly seen as most morally problematic, that is ‘lifestyle changes’ and ‘low-population’) could be implemented in morally permissible ways. By contrast, the less controversial ‘low non-CO₂’ and ‘agricultural intensification’ pathways actually deserve more ethical scrutiny. Therefore, scenarios involving lifestyle changes and population measures should not be prematurely dismissed. Nor should even the most commonly accepted strategies escape closer ethical scrutiny.

Whilst the first two papers address issues regarding the modelling and the assumptions surrounding CDR technologies, the next two papers discuss the responsibilities of the actors who could potentially implement CDR technologies, and the question of how the costs and benefits of CDR technologies should be distributed. Kian Mintz-Woo (Reference Mintz-Woo2023) identifies what he calls a ‘needs-efficiency trade-off’. There are two possible reasons to develop CDR, Minz-Woo argues. The first justification is the potential contribution that CDR technologies can make to global mitigation goals. The second is that investment in these technologies can contribute to development in the region where they are located. However, as Minz-Woo points out, these two justifications lead to different conclusions about where CDR measures should be implemented. If contribution to mitigation (efficiency) is taken as the primary concern, then implementation should be done in countries with the best prospect of success. Minz-Woo contends that successful deployment is most likely in Western industrialised nations, due to greater human capital and scientific understanding. However, if developmental concerns are primary (need) then CDR should be developed and deployed in developing regions of the world. This is because where developing countries are dependent on carbon-intensive industry and infrastructure, requiring them to simply reduce emissions could deepen socio-economic injustices. Using CDRs, Minz-Woo argues, offers the opportunity to ‘offset’ GHG emissions that are needed for economic development. Policy-makers should carefully consider whether they wish to pursue ‘efficiency’ or ‘need’ as the practical implications are very different.

Another conflict between efficiency and fairness is addressed by Lenzi et al. (Reference Lenzi, Schübel and Wallimann-Helmer2023). They outline the moral dilemma between holding carbon majors responsible for their historical contributions to climate change, and the prospect of their being among the best placed to offer certain forms of CDR (i.e. BECCs and DACCs). As the storage of carbon dioxide is likely to be very profitable, carbon majors thus stand to benefit from supplying fossil fuels and offsetting or cleaning up the carbon dioxide that results from their use. Accordingly, Lenzi et al. argue that from the perspective of ideal theory, carbon majors are not entitled to keep the profits from CDR usage until they have addressed their historical responsibility for causing climate change. This conclusion may be resisted under non-ideal circumstances, where it might be permissible to allow carbon majors to profit from CDR, if no other agents have the technical or institutional capacity to develop or use it. However, they argue that in more favourable circumstances, including those of most OECD countries, it remains impermissible to reward carbon majors for undertaking CDR until they compensate for their historical climate responsibilities. Additionally, Lenzi et al. echo the arguments of the first two papers: assumptions about the feasibility of development and deployment must be made explicit.

The papers in this collection thus present arguments showing how the development and use of CDR technologies may – or may not – be in line with climate justice and present conclusions addressing both policymakers and modellers. The discussion such questions is crucial in navigating the balance between technological possibilities, political demands, and moral requirements.